A ring laser gyroscope requires one or more adjustable mirrors in order to facilitate operation at the atomic gain center of the transition used for lasing. This is usually done in a closed servo fashion, where the tuning error or pathlength distance to the atomic gain center is measured and reduced to zero by translating the control mirror perpendicular to its mirrored surface. Such control mirror movement is known and has been described in, for example, U.S. Pat. No. 4,585,346, by the same inventor and assigned to the same assignee as the instant invention.
Such high accuracy ring laser gyroscope--of the type used for inertial navigation of aircraft, ships and missiles, etc.--usually uses long radius spherical mirrors, sometimes in combination with flat mirrors, in order to minimize the effects of back-scatter. Back-scatter, from the mirrors, is to some extent unavoidable, and its effects are to increase the noise or random walk of the instrument.
The choice of long radius mirrors decreases the laser acceptancel angle and therefore, excludes some of the back-scattered light. Unfortunately, the use of long radius mirrors makes the ring laser gyroscope quite sensitive to angular distortions. The mechanism for bias shift, as a function of angular distortion of the laser path, is rather involved, but is in essence comprised of sensitivity for Langmuir flow effects in the laser plasma in conjunction with imperfectly located gain bores.
The present day adjustable mirrors, for the ring laser gyroscope, are commonly translated by means of a piezoelectric transducer. The piezoelectric material used is usually lead-titania-zirconate, commonly referred to as PZT. In moving the control mirror of a pathlength controller in an in/out fashion parallel to its mirrored surace, these well-known piezoelectric transducers also, unfortunately, affect the control mirror with a moment. Such an unintentional moment is emanated from the less than perfect distribution of piezoelectric domains in the piezoelectric material. In the prior art control mirrors, this unintentional moment causes the control mirror to tilt a small, but unavoidable, amount, which in turn causes the ring laser gyroscope to respond with an indication of a rotation of the instrument about its input axis, even though no such input rotation has taken place.